Insulating body



Feb. 14, 1933. PAS-EN ET AL 1,897,818

INSULATING BODY Y Filed April 9, 1951 3 Sheets-Sheet l L hwy@ Feb. 14,1933. P. PAscHEN E'r AL INSULATING BODY s sheets-sheet A 2 Filed April9, 1931 Patented Feb. 1.4, 1933 UNITED STATES PATENT OFFIC PAUL PASCHEN,HANS RITTER, GEORG STAUBER, WILHELM GEBHARDT, AND KARL SCHMIEDEL, FNUREMBERG, GERMANY, ASSIGNORS TO SIEMENS-SCHUCKERT- WERKEAKTIENGESELLSCHAFT, OF BERLIN-SIEMENSSTADT, GERMANY, A. CORPORA- TION OFGERMANY INSULATING BQDY Application led April 9, 1931, Serial No.528,846, and in Germany April 14, 1930.

Our invention relates to insulating members and more particularly to aninsulating member made of ceramic material, for ex-l ample, ofporcelain, for electro-technical purposes having a wall or walls forseparating and insulating from one another two objects of differentialpotential. The insulating member is composed of several parts, whichform component portions of said separating and insulating wall. Theseportions are according to the invention joined dielectrically tight toone another, i. e. they are joined to one another in such a manner thatthe dielectric strength at the joint is substantially the same as at theother parts of the Wall; one of the principal features and greatadvantages of the invention is, that at the joint the thickness of theseparating wall is not increased. The joint used according to ourinvention extends through the wall, i. e. from one side of the wall tothe other, as distinguished from an overlapping joint usually employedin the art in joining insulating members used for the aforementionedseparation purposes. Accordingly the definition of the jointpasextending through the wall used in the annexed claims should belunderstood in the light of the foregoing explanation.

Our invention will be described with reference to the accompanyingdrawings, illustrating examples for carrying youtthe invention.

Fig. 1 shows a view of a high-tension instrument current transformer,partially in section.

Fig. 2 shows a longitudinal section through the same instrumenttransformer taken on the plane A-B of Fig. 1.

Fig. 3 shows a high-tension instrument potential transformer inlongitudinal sec'- tion.

Figs. 4 and 5 show another constructional 'form of a measuringtransformer, in two vertical sections at right angles to each other.

Figs. 6 and 7 show a high-tension instrument potential transformer, intwo vertical sections at right angles to each other.

Fig. S shows a suspension-type insulator in longitudinal axial section;

Fig. 9 shows a high-tensi0n condenser in longitudinal axial section; and

Fig. 10 represents a graph, showing the dielectric strength ofinsulating material joints as a function of the joint thickness. Thedifferent modifications shown and described are merely examples of thetypes of electric device in which our invention is of particularadvantage, and should not be understood as limiting the invention tothese types.

Referring to Figs. 1, 2, on the supporting insulator of tubular shape 1with the foot 2, the metallic base 3 is fixed. On this base is mountedthe laminated three-legged iron core 4, the middle leg of which passesthrough the opening 5 of the annular porcelain receptacle or casing 6.In the annular space inside the receptacle the low-tension winding 7 isplaced, and on its outside, the high-tension winding 8. The connectingleads 9 of the winding 7 are led out through the tubular insulatorextension 10, the axis of which is parallel to the axis ofV the annularreceptacle 6 and to the axis of the supporting insulator l. Theinsulator 10 is disposed inside the tubular supporting insulator l. Oneof the ends 11 of the winding 8, is electrically connected with the ironcore 4 at which the angle-shaped member 12 is provided, serving as theterminal for that winding end; the other winding end 13 leads to theangle-shaped terminal 14, which is also fastened to the iron core 4, butis insulated therefrom by interposed insulating material l5.

The annular receptacle 6 consists of two parts', of the annularreceptacle proper hav- 90 from actual tests,

ing a U-profile and of a flat annular cover 60, the parts being joinedtogether in a dielectrically tight manner. For this purpose at the joint61 the two parts are carefully plane ground. The grinding must be sofine that the joint is extremely close and constitutes practically auniform surface-to-surface contact throughout the joint width. It ismost essential to make the joint so close that its-thickness in thedirection at right angles to the joint surfaces, caused by possiblyremaining minute uneven portions in the surfaces, amounts to not morethan 1/ 100 to 1/200 mm. We have found that at such closeness of thejoint its dielectric strength suddenly rises and rapidly increases withfurther reduction in thickness. It is therefore better to make the jointstill thinner than 1/200 mm., and for instance a thickness of from oneto two microns is preferable.

ln Fig. 10 is shown a graph which is made and which shows that above athickness of 0.02 mm. the joint has a relatively small influence uponthe dielectric strength. Below a joint thickness of 0.01 mm. however thedielectric strength very rapidly increases to values, which are of theorder of that of the high class insulating materials themselves. We havefound in some instances with a joint thickness which was with certaintybelow 0.005 mm. break downs only at from 122-18 kilovolt occur. Thematerial was a glass plate 4 mm. thick joined by a butt joint accordingto theff' present invention. At one place wher'gathe' joint waspractically a continuous close contact for an appreciable distance, vthejoint broke down only at 30 kilovolt, at which also the solid portion ofthe glass plate broke down. These tests have proven that with asuiiciently finely finished joint, the dielectric strength of thematerial itself can be attained for the joint, or at least a strengthwhich is of the order of the material strength.

It is advisable to join together the surfaces of the parts of theinsulating member meeting at the joint by means of a very thin layer, orfilm of material of great dielectric strength, such as resin, asphalt,tar etc. The film has the object of filling out possibly remaininginequalities in the surfaces of the vments, as set forth in the claims,as distinjoint. lt will be noted that while in this particularmodification the joint surface extends along one of the surfaces of oneof the component elements of the joint (namely along the inner annularsurface of rin the joint nevertheless extends through the wall of thecasing formed by the two eleguished from an overlapping joint. Also,

as stated in the annexed claims, the two component elements of the jointabut against one another-in 'the sense that pressure exerted on oneelement will increase the pressure on the joint surfaces, which wouldnot be true if for instance annular cover 60 were inserted into casing6, forming a joint parallel to the cylindrical wall of the casing.

It is particularly advantageous to tighten dielectrically the joint oftwo such component parts by means of artificial resin capable of beinghardened. For this purpose the surfaces of the joint are, for example,dipped into a solution of resin or into melted resin, the partsbeingpreferably warmed at the same' time. To harden the resin, the partsare then put together and, with the joint surfaces forcibly pressedagainst each other, are placed in a heated chamber, where they remain alength of time depending upon the degree of hardness desired. Theresulting product obtained with this method is of the higher quality,the thinner the resin film inthe joint, i. e., the better the parts ofthey insulating member fit together, and the slower the hardeningprocess is carried out. After the hardening is finished, the wall of theinsulating body, as tests show, possesses along the joint the samedielectric strength as at the integral wall portion. According to testscarried out, a joint in a wall of about l cm'. thickness at the joint,has a puncture strength of 40 to 50 kv. and more, i. e., it has the samepuncture strength as the integral wall portion. The resin not onlytightens the joint dielectrically, but also makes the connection of theparts of the insulating member mechanically so strong that it is able tooffer an extremely great mechanical resistance.

So-called plastic shellac is also particularly suitable for tighteningthe joint dielectrially. It is produced by saponification of naturalshellac or artificial shellac, the shellac molecule or a part of thesemolecules being decomposed into components, which form a thick liquidmass, which dissolves the molecules which were not decomposed. Thisplastic shellac may according to requirements be hardened to a greateror lesser degree. The hardening takes place quickly at about 180 deg.C., but the resulting joint is in that case not as good as it might be.For joining, the surfaces are dipped into the shellac mass, then placedupon one another and, with or without the application of pressure, putinto a chamber having a relatively low hardening temperature, 100 deg.C. or less. Here again, the resulting product is all the better, theslower and the more careful the hardening of the shellac is'carried out.

The great dielectric and also mechanical strength obtained with thismethod of joining the parts of the insulating member, permits theplacing of the joint, if desired, into the electrically highly stressedportion of the insulating member, without danger of breakjoint ispractically just as ing down. There is no necessity for making the wall'of the insulating member thicker at the joint, as the dielectricstrength of the reat as that of the insulatin member itsel For the a ovenamed reasons, joints of component parts of insulating bodies .of widelyvar ing forms may be placed, where it is most avorable for the assemblyof the insulating member and of the electrical apparatus, and where itpermits the most favorable shape for the individual parts of theinsulating member, and not where it is furthest removed from dielectricstresses as is customary in the present day art.

This is particularly important with porcelain as insulating mategial, ashere ver complicated forms may be subdivide into simple componentelements which are easily shaped and do not suffer any distortion infiring. In this manner, it is also possible, as shown in Figs. 1 and 2,to produce porcelain receptacles or casings of very great dielectricstrength which are adapted to the shape of, and come into close contactwith the windings of an electrical'apparatus. Owing to these electricqualities and to the small s ace which such an article requires, partsof) electrical apparatus having a very great difference in potential canbe placed very close to one another in a small s ace. With the currenttransformer shown in lgs. 1 and 2, this' has the special advantage thatthe mean length of the path in the iron core 4 can be made very short, afeature which according to the known laws ensures great accuracy ofmeasurement with such a current transformer. s

To avoid low discharges, the insulating casing 6 in Figs. 1 and 2 isprovided both, inside and outside with a conducting lining which alsocovers the edges of the joint 61. The lining should extend also over atleast a part of the length of insulator 1Q, containing the connectinglow otentlal wires, Over the electrical partso the instrumenttransformer, a cover 16 1s placedand provided with openings 17 for aircirculation which cools the windings and the insulating casing.

In F ig. 3, the insulating casing of a potential transformer is composedof two approximately symmetrical halves 62, 63. 61 is the joint throughthe Wall of the vassembled casing, in this case forming a true buttjoint. Into one half 63, part 18 of the high-tension winding is placed,and the other half 62 contains the other part 19 of that Winding.` Theconnecting wlres 20, 21 of the high-tension windin are led out throughinsulators 22, 23. he insulators may be formed integral with thepertaining part of the casing, as shown in the left hal of Fig.l 3, butmay also consist of a se arate Ielement joined to the casing in a dieectri- 'a dielectrically tight joint 29. 4 is cally tight manner by thebutt joint 610. The numeral 24 indicates the low-tension winding.

The space inside of the insulating members, 62, 63 not occupied bywindings and leads may be filled with oil or insulating compound torevent corona occurrence in these spaces. he vertical insulator sleevesmay then serve at the same time as expansion vessels for the fillingcompound.

In Figs. 4 and 5, corresponding substantially to Figs. 1 and 2, thesupporting in- ,sulator consists ofv a pot-shaped insulating member 25,onto the bottom of which the insulating casing 26 is joined in adielectrically tight manner. The joint is indicated by 27. 28 is thecover of the casing, which is also joined to the body 26 of the casin byere again the iron core, 7, 8 are the two windings. The threaded bolts30, which also serve as connecting conductors for the winding 7, passthrough the joint 27 By means of nuts 31 the insulating parts 25, 26 canbe tightly drawn together at the joint 27.

Whilst in Figs. 5 and 4 the electrically active transformer parts arepositioned on the outer side of the pot-shaped supporting insulator 25,in Figs. 6 and 7 they are accommodated inside the pot. Here, again, 4 isthe iron core, 7, 8 are the two windings, 28 is the cover of the casing,26 is the part of the casing with a U-shaped profile. The casing hasopenings 260 directed downwardly' in the drawings. The wires forconnecting up the winding 7 are passed through these openings. The part26 is integral with the insulator 32; the bottom of the part 32 isprovided with an opening for introducing the sheets for the iron core 4,the sheets being placed alternately from the top and the bottom. Theopening is closed dielectrically tight through the plate 33 by means ofthe joint 34. A special feature of this form of instrument transformeris that its dimensions and height are particularly small.

In Fig. 8, a suspension insulator of the so-called cap type isillustrated. 35 is the stem with the head 36. The cap consists of thepot 37 and the cover 38 threaded into the pot and provided with thesuspension eye.39. The insulating member is composed of the supportingcup 40, the cover 41 and the bell 42. At the joints 43, 44, theinsulating parts are ground and tightened dielectrically against eachother in the manner described above. The parts 40, 41 are mcchanicallyheld together by the parts 37 and 38, a pad 45 being placed between thepressing and pre ised parts. The parts 42 and 40 are clamped together onstem 35, by head 36 and the tightening nut 46 threaded onto stem 35.

This embodiment of the invention shows that for suspension insulatorsthe insulating member-'can also be subdivided in the manner the mostfavorable for assembling and for the purpose iu question, and thatthrough the subdivision simple, easily producible porcelain parts areobtained, instead of the otherwise complicated forms of insulators usedheretofore.

kThe advantage of the insulator shown iu Fig. 8 consists in theexistence of only compression stresses in the insulating material and inthat it is impossible for the stem to fall out of the insulating memberand for the insulating member iu the cap to become loose. As the joints43, 44 have the same dielectric strength as the wall of the insulatingmember, they may without hesitation be positioned at places which areexposed to great electric stresses.

A further application of the invention is shown in Fig. 9. The drawingsillustrate a high-tension condenser with porcelain as a dielectric andin which a large capacitance is to be accommodated in a small space. Byintroducing joints the very complicated insulating member is heresubdivided ink a series of simple trays or shields 47 as in the bottompart, or of flat bottle-shaped members 48, as in the middle part, or ofshort pieces of tube with broad flanges 49 at their ends, as in theupper part. The joints are indicated at 50. Here again, the joints aremade dielectrically tight in the described manner. The separate parts ofthe insulator arev mechanically held together by the threaded bolt 51and the nuts 52.

The bolt 51 is passed through the leadingin insulator 53 to the one pole54 of' the condenser. lVith exception of the surfaces of the jointsthemselves, the component parts of this ventire insulating structure arecovered, as shown by the heavy contour lines, inside and outside withelectrically conducting coatings of, for example, graphite andwaterglassor the like, adhering closely to the insulating material. Theinner coatings are connected to thelbolt 51, and the outer coatings tothe metal housing 55 which is in connection with the other terminal 56of the condenser.

The condenser according to Fig. 9 has the advantage over knowncondensers also built up of parts having the form of .plates or trays,that the whole surface of the individual elements, of which theassembled insulating body is composed, including the joint edges, can beutilized for placing the coatings, whereas in the known condensers thejoint edges must remain free, and the width of these free or blankzones,particu larly in high-tension condensers, very great,

so that for a certain capacitance much more' -insulating material 1srequired. Another considerable advantage consists in the avoidmg ofsurface leakage currents over the assasi edges of the componentinsulator parts, because the entire insulating body, in spite of itscomplicated form, forms a completely closed vessel which separates theinner and outer coatings. As the dielectric of' the condenser consistsof porcelain, which material advantages in cases in which complicatedforms of insulating elements are required. lVhen' windings, conductingcoatings, supporting parts and the like are to be introduced into thehollow spaces of the members after the baking, the invention is'ofinestimable value. If an insulating member consisting of several partsis suitably assembled, it will always be possible to avoid a complicatedthreading through of` wire for windings, and to ensure that allsupporting parts are well cemented-in and that no parts of theinsulating member are subject to mechanical tension stresses.Furthermore, our invention also renders it possible to use superiordielectrics, such as porcelain, in fields of electrical engineeringwhere the prior art has usually employed oil, insulating compound,layers of brous material and the like for insulating purposes.

l/Ve claim as our invention:

l. A body of electric insulating material, having a wall for separatingtwo spaces containing objects of different electrical potentials, saidwall comprising at least two component portions/having a joint extendingthrough the wall and having the contacting joint surfaces suiiicientlyfinely finished to form substantially a uniformly closesurface-to-surface contact throughout the extent of the joint, and toimpart to said joint a dielectric strength ofthe order of that of theadjacent material.

2. A body of electric insulating material,

having a wall for separating two spaces.

containing objects ofdiferent electrical potentials, said wallcomprising at least two component portions having a joint extendingthrough the wall and having the contacting joint surfaces finely inishedto form substantially a uniformly close surface-tosurface contactthroughout the extent of the joint, said joint being not thicker than 1/100 3. A body of electric insulating material, having a wall forseparating two spaces containing objects of different electricalpotentials, said wall comprising at least two separate portions abuttingagainst one another to form a joint extending through the wall, andhaving their contacting' surfaces sulficiently finely finished to formsubstantially a uniformly close surface-to-surface contact throughoutthe extent of the joint and to impart to said joint a dlelectricstrength of the order of that of the adjacent material, and a film ofdielectrically highly resistive binding material disposed between said jointy surfaces.

4. A body of. electric insulating material, having a Wall forvseparating two spaces containing objects of different electricalpotentials, said wall comprising at least two separate portions abuttingagainst one another to form a oint extending through the wall, andhaving their abutting surfaces sufficiently finely finished to formsubstantially a uniformly close surface-to-surface contact throughoutthe extent of the joint and to impart to said joint a dielectricstrength of the order of that of the adjacent material, and a. thin filmof artificial resin disposed between the jointl surfaces.

5. A body 0f electric insulating material, having a wall for se aratingtwo spaces containing objects of different electrical potentials, saidwall comprising at least two separate portions abutting against oneanother to form a joint extending through the wall, and having theirabutting surfaces sufficiently finely finished to form substantially auniformly close surface-to-surface contact throughout the extent of thejoint and to impart to said joint a dielectric strength of the order ofthat of the adjacent material, and a thin film of plastic shellacdisposed between the joint surfaces.

6. A body of electric insulatlng material, having a wall for se aratingtwo spaces containing objects of different electrical potentials, saidwall comprising at least two separate portions abutting against oneanother to form a joint extending through the wall, and having theirabutting surfaces finely finished to form substantially a uniformlyclose surface-to-surface contact throughout the extent of the joint, anda thin film of dielectrically highly resistive binding material disposedbetween the joint surfaces, the thickness of the complete joint beingnot greater than l/lOO mm.

7. A body of electric insulating material', having a wall for separatingtwo spaces containing objects of different electrical potentials, saidwall comprising at least two separate portions abutting against oneanother, and having their abutting surfacescontaining objects ofdifferent electrical potentials, said wall comprising at least twoseparate portions abutting against one anot er to form a joint extendingthrough the wall, and having their abutting surfaces sufficiently finelyfinished to form substantially a uniformly close surface-to-surfacecontact throughout' the extent of the joint and to impart to said jointa dielectric strength of the order of that of the adjacent material, theentire surface of the joint being located in one plane. j

9. A body of electric insulating material, having a wall for separatingtwo spaces containing objects of different electrical potentials, saidwall comprising at least two component portions forming a jointextending through the wall and having their abutting joint surfacessufficiently finely ground to form substantially a uniformly closesurface-to-surface contact throughout the extent of the joint and toimpart to said joint a dielectric strength of the order of that of theadjacent material.

10. A body of electric insulating material, having a wall for separatingtwo spaces containing objects of different electrical potentials, saidwall comprising at least two component portions forming a jointextending through the wall and having their abutting joint surfacessufficiently finely ground to form substantially a uniformly closesurface-to-surface contact throughout the ex-4 tent of the joint and toimpart to said joint a dielectric strength of the order of that of theadjacent mate ial, and a thin film of dielectrically highl resistivebinding material disposed between the ground surfaces.

1l. A body of electric insulating material, having a wall for separatingtwo spaces containing objects of different electrical poten-l tials,said wall comprising at least two component portions forming a jointextending through the wall and having their abutting joint surfacessufiiciently finely ground to form substantially a uniformly closesurface-to-surface contact throughout the extent of the joint and toimpart to said joint a dielectric strength of the order of that of theadjacent material, the entire joint surface being located in one plane,and a thin film of dielectrically highly resistive binding materialdisposed between the joint surfaces.

12. A body of electric insulating material, having a wall for separatingtwo spaces containing objects of different electrical potentials, saidwall comprising at least'two 'component portions forming a jointextending through the wall and having their abutting joint surfacesfinely ground to form substantially a uniformly close surface-to-surfacecontact throughout the extent of the joint, the entire joint surfacebeing located in oneplane, and a thin film of dielectri- 10 through thewall, and' having their abutting joint surfaces? `'sufficiently finelyfinished to form substantially a uniformly close surface-to-surfacecontact throughout the extent of the joint and to impart to said joint adielectric strength of the order of that of the adjacent material.

14. A body of porcelain having a wall for separating two spacescontaining objects of ,different electrical potentials, said walllcomprising at least two component portions orming a joint extendingthrough the wall, and having their abutting joint surfaces sufficientlyfinely ground to form substantially a uniformly close surface-to-surfacecontact throughout the extent of the joint and to impart to said joint adielectric strength of the order of that of the adjacent material.

15. An electrical insulating body of ce ramic material having a wall forseparating two spaces containing objects of different electricalpotentials, said wall comprising at least two separate portions abuttingagainst one another to form a joint extending through the Wall, andhaving the abutting joint surfaces finely ground to form substantially auniformly close surface-to-surface contact throughout the extent of .thejoint, and a thin film of dielectrically highly resistive bindingmaterial between the joint 4 surfaces, the thickness of the joint beingnot more than 1/100 mm.

16. An electrical insulating body of ceramic material having a wall forseparating two spaces containing objects of different electricalpotentials, said wall comprising at least two separate portions abuttingagainst one another to form a joint extending lthrough thei wall, andhaving the abutting joint 'surfaces finely ground to form substantiallya uniformly close surface-to-surface contact throughout the extent ofthe-joint, and a thin film of'dielectrically highly re sistive bindingmaterial between the joint surfaces, the thickness of the joint beingnot more than 1/ 100 mm., said ground joint sur-- faces extending in oneplane.

17. An electrical insulating body of ceramic material having a wall forseparating two spaces containing objects of different 69 electricalpotentials, said wall comprising at least two separate portions abuttingagainst one another to form a joint extend'- ing through the wall, andhaving the abutting joint surfaces finely ground to form substantially auniformly close surface-tosurface contact throughout the extent of thejoint, and a thin film of dielectrically highly resistive bindingmaterial between the joint surfaces, the thickness of the joint beingnot more than 1/100 mm., the transverse extent of thel joint surfacesbeing approximately in parallelto the lines of electric stresses, and aconducting coating on at least one side of said wall, covering saidjoint.

18. A body of electric insulating material, having the form of a casinorfor elec-- trically insulating objects of different potentia'ls disposedinside and outside the casing, said casing comprising at least twocomponent parts assembled to form a joint runnin through the casingwall, the abutting joint surfaces being sufficiently finely ground to'form a plane, substantially uniformly close surface-to-surface contact,throughout the extent of the joint and to impart to said joint adielectric strength of the order of that of the adjacent material.

19. A body of electric insulating material, having the form of a casingfor electrically insulating objects of different potentials disposedinside and outside of the casing, said casing comprising at least twocomponent parts abutting against one another to form a joint runningthrough the casing wall, the abutting joint surfaces being sufficientlyfinely ground tol form a plane, substantially uniformly close'surface-to-surface contact throughout the extentof the joint and toimpart to said joint a dielectric strength of the older of that of theadjacent material, and a thin film of dielectrically highly, resistivebinding material disposed between the joint surfaces.

20. A body of electric insulating material, having the form of a casingfor electrically insulating objects of different potentials disposedinside and outside of thev casing, said casing comprising at least twocomponent parts abutting against one another to form a joint runmng'through the casing wall, the abutting joint surfaces being finelyground to form a plane, substantially uniformly close surface-to-surfacecontact throughout the extent of the joint, anda thin film ofdielectrically highly resistive binding material disposed between thejoint surfaces, and forming a joint not thicker than 1/100 mm.

21. A body of electric insulating material, having the form of a casingfor electrically insulating objects of different potentials disposedinside and outside of the casing, said casing comprising at least twocomponent parts abutting against one another to form a joint runningthrough the casing wall, the abutting joint surfaces being sufficientlyfinely ground to form a plane, substantially uniformly closesurface-to-surface contact throughout the extent of the joint and to isoimpart to said joint a dielectric strength of the order of that of theadjacent material, and a conducting coating on at least one side of thecasing wall, covering said joint.

22. An electrically insulating body of ceramic material having the formof a hollow ring, for electrically insulating objects of differentpotentials disposed within thewalls of said ring and outside thereof,said ring comprising at least two component parts abutting against oneanother to form a joint running through the `ring walls, the abuttingjoint surfaces being finely ground to form a plane, substantiallyuniformly close surface-to-surface contact throughout the extent of thejoint, ,and a thin film of dielectrically highlyl resistive bindingmatea joint at least as close as 1/10() mm., and a thin film ofdielectrically highly resistive binding material, disposed between thejoint surfaces for equalizing the remaining minute uneven portions inthe joint surfaces. v

24. An instrument transformer having an annular casing of ceramicelectrically insulating material, a4 first winding disposed" within theannular casing portion, and a second winding disposed concentricallywith the first winding, but outside of one of the casing walls, and amagnet core extending centrally through said casing, for magnet- `icallyinterlinking said two windings, said casing comprising at least twocomponent portions forming a joint running through the casing wall, theabutting joint surfaces being finely ground fiat to produceasufficiently close and uniform' surface-to-surface contact, to form ajoint at least as close as 1/100 mm., anda thin film of dielec tricallyhighly resistive binding material, disposed between the joint surfacesfor equalizing the remaining minute uneven portions in the jointsurfaces.

25. An instrument transformer having an annular casing of ceramicelectrically insulating material, a first winding disposed within theannular casing portion, and a second winding disposed concentricallywith the first winding, but outside of one of the casing walls, and amagnet core extending centrally through said casing. and an insulatingsleeve on said casing extending substantially in parallel to the casingaxis, and forming an outlet for said first winding, said casingcomprising at least two component portions forming a joint runningthrough the casing wall,the abutting joint surfaces being finely groundflat to produce a sufiiciently close and uniform surface-t0- surfacecontact, to form a. joint at least as close as 1/100 mm., and a thinfilm of dielectrically highly resistive binding material, disposedbetween the joint surfaces for equalizing the remaining minute-unevenportions in the joint surfaces.

26. An instrument transformer having an annular casing of ceramicelectrically insulating material, a first winding disposed within theannular casing portlon, and a second winding disposed concentricallywith the first winding, but outside of one of the casing walls, and amagnet core extending centrally through said casing, and an insulatingsleeve on said casing, extending substantially in parallel. to thecasing axis, and forming an outlet for said first winding, said casingcomprising at least two component portions forming a joint runningthrough the casing wall, the abutting joint surfaces being finely groundflat to produce a sufficiently close -and uniform surface-tosurfacecontact, to form a joint at least as close as 1/100 mm., and a thin filmof dielectrically highly resistive binding material, disposed betweenthe joint surfaces for equalizing the remaining minute uneven portionsin the joint surfaces, and a tubular base of insulating material forsupporting said casing and surrounding said insulator sleeve.

27. An electrical insulator of ceramic material comprising an annularcasing of U- shaped cross-section for electrically insulating from oneanother objects disposed inside of the casing and outside thereof, andan annular cover for said casing resting upon the wall ends of thecasing, the contacting surfaces of said two elements being finely groundin oneplane to produce a sufficiently close and uniformsurface-to-surface contact to form a joint of a dielectric strengthsimilar to that of the integral wall portions of said two elements.

28. An electrical insulator of ceramic material comprising an annularcasing of U- shaped cross-section for electrically insulating from`l onelanother objects disposed inside-of the casing and outside thereof, andan annular cover for said casing resting upon the wall ends of thecasing, the contacting surfaces of said two elements being finely groundin one plane to produce a sufficiently close and uniformsurface-to-surface contact toform a joint of a dielectric strengthsimilar to that of the integral wall portions of said two elements, anda thin film of dielectric:l highly resistive binding material disposedbetween the joint surfaces for equalizing the remaining minute unevenportions in the joint surfaces.

In testimony whereof we aiix our signa,-

tures.

PAUL PASCHEN. HANS RITTER. GEORG -STAUBER WILHELM GEBHARDT. KARLSCI-IMIEDEL. v

